Supports conventionally used for magnetic recording tapes include polyester films such as polyethylene terephthalate films, polyethylene naphthalate films, etc.; polyolefin films such as polypropylene films, etc.; cellulose derivative films such as cellulose triacetate films, cellulose diacetate films, etc.; films such as polycarbonate films, polyvinyl chloride films, etc.; papers; foils of a non-magnetic metal such as copper, aluminum, zinc, etc.
Among the aforementioned supports, polyolefin films show large expansion and contraction even by a small deviation of tension and show poor jitter characteristics due to the low tensile elasticity. The foils of a non-magnetic metal such as copper, aluminum, zinc, etc., have high tensile elasticity but are liable to be cut and show a weak tear resistance due to a low percentage elongation at break. These films have a fault of readily causing plastic deformation. Cellulose derivative films and paper supports have the fault that the hygroscopic coefficient of expansion is large. Polycarbonate films are poor in organic solvent resistance and polyvinyl chloride films are low in heat resistance.
Polyester films, in particular, polyethylene terephthalate films, have recently been used as supports for magnetic recording tapes, since they have relatively few faults and have balanced characteristics, for example, substantially equal Young's modulus in the lengthwise and transverse directions. More specifically, polyethylene terephthalate films having a thickness of from about 8 to 16 .mu.m are used for home video cassette tapes; e.g., Beta-Format (trademark of Sony Corp.), VHS tape (trademark of Victor Corp. of Japan, Ltd.), etc., and polyethylene terephthalate films having a thickness of from about 4 to 14 .mu.m are used for audio cassette tapes.
It has been proposed to reduce the thickness of a base film for prolonging the recording period of time of a home video tape but if the thickness of the base tape is reduced, the tape is liable to undergo expansion and contraction with the deviation of tension during the running of the tape, which results in jitter undesirable for television images. Also, video tapes using such base tapes having reduced thickness are liable to undergo problems such as crumpling and folding during running of the video tape.
It has also been practiced to reduce the thickness of an audio cassette tape for prolonging the recording time, but as the thickness of the base tape is reduced, the audio recording tape is liable to undergo distortion of sound, i.e., so-called wow and flutter. Such audio tape also shows such undesirable faults as folding of tape at the edge portion thereof, a reduction in smoothness due to the permanent distortion of the edgeportion of the tape, and the occurrence of a large amount of curling.
The deviation in tension of a magnetic recording tape in the lengthwise direction of the tape caused by friction and fluctuation of frictional force during running of the tape causes stretching vibrations, which results in jitter (video tape) and wow and flutter (audio tape). For overcoming this difficulty, it is desirable that the elongation percentage per unit tension (e.g., elongation percentage/100 g load) be kept constant even if the thickness of the support is reduced. That is, even if the thickness of a tape is reduced, the above-mentioned difficulty can be overcome by keeping the Young's modulus of the tape in the lengthwise direction high in inverse proportion to the thickness.
The occurrence of folding and crumpling in the transverse direction depends on the bending strength of the tape in the transverse direction. Since the bending strength is proportional to Young's modulus.times.(thickness).sup.3 in the transverse direction, if the thickness of the tape is reduced from, for example, 15 .mu.m to 10 .mu.m and the Young's modulus is same, the bending strength is reduced to only 8/27ths of the original bending strength. It has, therefore, been found that for preventing the occurrence of folding and crumpling when the thickness of a tape is reduced, it is necessary to greatly increase the Young's modulus of the tape in the transverse direction.
Therefore, for overcoming the aforesaid difficulties in case of reducing the thickness of a base tape for magnetic recording tape, it is necessary to increase both the Young's modulus in the lengthwise direction and the Young's modulus in the transverse direction.